The Subaru engineers bestowed a stout 20mm sway bar on the 02-to-early-03 WRX sedan. You can fit this to your WRX wagon (which comes with a 17mm rear bar) or TS wagon (which has a 13mm). Or even use it as an affordable upgrade for a late-model WRX sedan, which also has a 17mm rear bar.
The black color makes it very stealthy. Heck, it's a stock part. Nobody will be the wiser.
Kit includes sway bar and two 20mm rubber bushings.